Getting color from a face/polygon via python to select the corresponding face/polygon
Welp, I don't really like to answer my own questions. But I found the solution and I wanted to share it with you :)
bm = bmesh.new()                          #Generate new bmesh object
bm.from_mesh(obj.data)                    #Assign the model mesh
    for face in bm.faces:                 #goes through the faces
        for loop in face.loops:           #goes through the loops of each face
            #your condition here, eg:
            if loop[bm.loops.layers.color[0]] == Vector((1,1,1,1)): #the left side is the vertex color
                loop.vert.select = True   #to select the vertex if correct
            else:
                loop.vert.select = False  #a good addition. sometimes all vertices are selected from the beginning

So the trick is convertig it to a bmesh and go through that bmesh.
After everything is done, just throw it back to the bpy-mesh:
bm.to_mesh(obj.data)

And if you change the view to edit-mode you will notice that all the (in this case white) vertices are selected while the rest is deselected. Just make sure your object is in object-mode. At least, when giving returning the bmesh back to your mesh.
(And for all the newbies: &quot;obj.data&quot; stands for &quot;bpy.data.objects['x'].data&quot;, which is of course the mesh of the object :) )